1. Field of the Invention
This invention relates to a mold for cast molding a substrate for information recording medium and particularly, to a mold for casting having a preformat pattern of information pits.
2. Related Background Art
In the prior art is a substrate for information recording medium having an uneven preformat such as tracking grooves or pits for information address, etc., i.e., for optical information recording and reproduction. Such preformats have been formed on information recording medium substrates by (1) transferring an uneven preformat pattern of the stamper mold by the injection or hot press method when the substrate comprises a thermoplastic resin, (2) the so called the so called 2P method in which a photocurable resin composition is coated on a transparent resin plate comprising a thermosetting resin or a thermoplastic resin, adhering a stamper mold thereon, imparting an energy such as UV-ray from the transparent resin side to cure the resin composition and thereby transferring the replica of the stamper mold onto the transparent resin and (3) the cast molding method in which a monomer of a resin or a prepolymer containing a solvent is injected into a casting mold having an uneven preformat pattern corresponding to the uneven preformat and then cured to effect the pattern.
However, the thermoplastic resin substrate obtained by the injection method or the hot press method suffers from warping the substrate after molding or occurrence of optical anisotropy due to residual stress or orientation of molecules by the thermal hysteresis during molding.
On the other hand, the substrate obtained by the 2P method is problematic in that the polymerization initiator and the monomer used during photopolymerization are liable to remain in the resin after photopolymerization and affect the optical recording layer of the recording medium, thereby causing deterioration of the recording characteristis.
As the method for preparing a substrate for information recording medium free from these problems, there may be sometimes used the cast molding method discussed above and as disclosed in Japanese Laid-open patent publication No. 61-92450. The substrate obtained by this method is substantially free from pressure applied during molding as compared with the methods (1) and (2), and also thermoplastic resins, thermosetting resins, photocurable resins are available for use as resins. Therefore it is possible to select a resin which does not have deleterious influence on the recording layer.
However, the substrate obtained by the injection molding method had the problem that the tracking groove and the edge portion of the information pit are missing after mold release.
The damage to the substrate occurring during mold release may be considered to be due to the stress applied primarily on the edge portion through the adhesive force between the substrate and the mold for cast molding and thermal shrinkage of the substrate. Since this stress is liable to be applied on the edge portion of the information pit which is in the direction perpendicular to the tracking groove, because the tracking groove exists lengthy in the lateral direction of the substrate, damage is liable to occur in the information pit rather than in the tracking groove.
FIG. 3 shows a sectional plan view of a preformat on the substrate having the defect generated during cast molding by use of a mold for cast molding of the prior art as described above. Particularly noticeable between the tracking grooves 1' of the preformat are defects 8 such as formless shape or crack which damages the pit at the edge portion 2' formed in parallel to said tracking groove 1'.
Such defects may cause reading incapability or reading error during reproduction of the preformat signal, resulting in damage of reliability of the information recording medium. In other words, generation of these defects is a problem which is a great obstacle in preparing an information recording medium of high reliability. Particularly, when the yield of the damage free substrate becomes 50% or less, the production cost of the information recording medium becomes prohibitive.
On the other hand, in such an information recording medium that the recording layer is formed by coating of a dye, etc. on a substrate having an uneven preformat, the recording medium is penetrated into the pit for preformat during coating, and particularly at the portion where the pits are provided approximate to each other, the thickness of the recording layer at the convexity between the information pits of the substrate (hereinafter called the land portion) becomes thinner.
Since the reflected light quantity when a laser beam is irradiated is determined depending on the thickness of the recording layer, there has been involved the problem that no sufficient reflected light quantity can be obtained.
U.S. Pat. No. 4,209,804 discloses a recording carrier which performs recording of recording information by varying the length of the information region. However, when such recording is performed, the pit length becomes longer and the land portion between the pits becomes shorter, whereby the land portion is very susceptible to be damage during mold release in cast molding.